1. Field of the Invention
The present invention relates to an optical receiving apparatus and an optical transmission system.
2. Description of the Related Art
In recent years, there has been a growing demand for the introduction of next-generation optical transmission systems that can operate at a transmission rate of 40 Gb/s. Moreover, such optical transmission systems are expected to have the transmission distance and frequency usage efficiency equivalent to those of the systems operating at 10 Gb/s. In search of means to fulfill such demand, research-and-development (R&D) efforts are being made actively for the RZ-DPSK (return to zero—differential phase shift keying) or CSRZ-DPSK (carrier-suppressed return to zero—differential phase shift keying) modulation scheme which excels in the OSNR (optical signal-to-noise ratio) tolerance and nonlinearity tolerance compared with the NRZ (non-return to zero) modulation scheme having been employed in conventional systems of 10 Gb/s or below. Also, in addition to the above-mentioned modulation schemes, active R&D is being directed to phase-modulation schemes, such as the RZ-DQPSK (return to zero—differential quadrature phase-shift keying) or the CSRZ-DQPSK modulation scheme, which feature high frequency usage efficiency with narrow spectrum (See Reference (1) and Reference (2) in the following Related Art List, for instance).
FIG. 1 is a block diagram showing an exemplary structure of a conventional optical transmitting apparatus for transmitting optical signals by the use of an RZ-DPSK or CSRZ-DPSK modulation schemes of 43 Gbp/s. FIG. 2 is a block diagram showing an exemplary structure of a conventional optical receiving apparatus that performs receiving processings, such as demodulation, on the optical signals transmitted from the optical transmitting apparatus. In the transmission and receiving of optical signals by the RZ-DPSK or CSRZ-DPSK modulation and demodulation scheme, the optical intensity takes a 43 GHz clock waveform, and information is carried by binary optical phase.
As shown in FIG. 1, an optical transmitting apparatus 110 includes a transmission data processing unit 111, a CW (continuous wave) light source 112, a phase modulator 113, and an LN intensity modulator 114.
The transmission data processing unit 111 is provided with a function as a framer for framing inputted data, a function as an FEC (forward error correction) encoder for adding error correction codes, and a function as a DPSK precoder for performing a coding process reflecting information on a difference between the current code and the 1-bit preceding code. The phase modulator 113 modulates continuous light from the CW light source 112 by coded data fed from the transmission data processing unit 111 and outputs a DPSK-modulated optical signal, which is an optical signal, with constant optical intensity, carrying information on the binary optical phase. FIG. 3 is a graph showing a relationship between the optical intensity and the optical phase of a DPSK-modulated optical signal. The LN intensity modulator 114 performs an RZ-pulsing on the optical signal fed from the phase modulator 113. Note that an optical signal which is RZ-pulsed using a frequency (43 GHz) being the same as the bit rate and a clock driving signal having an amplitude equal to the extinction voltage (Vπ) is referred to as an RZ-DPSK signal. Note also that an optical signal which is RZ-pulsed using a frequency (21.5 GHz) being half of the bit rate and a clock driving signal having an amplitude twice as large as the extinction voltage (Vπ) is referred to as a CSRZ-DPSK signal. The optical signal RZ-pulsed by the LN intensity modulator 114 is transmitted to an optical transmission path 101.
Also, an optical receiving apparatus 120 shown in FIG. 2, which is connected to the optical transmitting apparatus 110 via the optical transmission path (optical fiber) 101, performs receiving processings on the (CS)RZ-DPSK signal. As shown in FIG. 2, the optical receiving apparatus 120 includes a VDC (variable dispersion compensator) 121, an optical amplifier 122, a delayed interferometer 123, a photoelectric converter 124, a reproduction unit 125, a received data processing unit 126, and a control unit 127. The optical receiving apparatus 120 performs a highly precise wavelength dispersion compensation by the variable dispersion compensator 121 disposed at the input end thereof because the wavelength dispersion tolerance in 43 Gb/s transmissions is only about 1/16 of that in 10 Gb/s transmissions.
The variable dispersion compensator 121 performs wavelength dispersion compensation on the (CS)RZ-DPSK signal sent through the optical transmission path 101. The optical amplifier 122 amplifies the power of the optical signal outputted from the variable dispersion compensator 121 to a predetermined level so as to compensate for the loss of light at the variable dispersion compensator 121 and outputs the amplified optical signal to the delayed interferometer 123. The delayed interferometer 123, which may be a Mach-Zehnder interferometer, for instance, produces two optical outputs from the inputted signal through an interference (delay interference) in which a component delayed by one bit time (23.3 ps in this case) and a component having been subjected to a 0-rad optical phase control are interfered with each other. That is, one of branch waveguides, which constitutes the Mach-Zehnder interferometer, is so formed as to be a propagation length, equal to 1 bit time, longer than the other of the branch waveguides. The photoelectric converter 124 is structured by a dual-pin photodiode that performs a differential photoelectric conversion detection (balanced detection) by receiving the two optical outputs from the delayed interferometer 123. The reproduction unit 125 extracts a data signal and a clock signal from the received signal which has undergone a balanced detection by the photoelectric converter 124. The received data processing unit 126 performs signal processings, such as error correction, based on the data signal and the clock signal extracted by the reproduction unit 125. The control unit 127 monitors the number of error occurrences detected in the error correction process at the received data processing unit 126 and performs a feedback control of the variable dispersion compensator 121 and the delayed interferometer 123 in such a manner as to minimize the number of error occurrences.
As a conventional technology related to the control of the variable dispersion compensator in an optical transmission system employing an optical modulation scheme such as (CS)RZ-DPSK, Reference (3) in the following Related Art List discloses a technology for monitoring the quality of optical signals without a demodulation process of received optical signals. Also disclosed in Reference (4) and Reference (5) in the following Related Art List, for example, are technologies for optimizing through a feedback control of the variable dispersion compensators or the like provided in the transmission, relay, and receiving sections, based on the transmission characteristics measured at the receiving end.